Media captionHome Secretary Theresa May: "There are issues that have been raised that will be looked at seriously"

Ministers will "look very seriously" at a report that suggests more than half of criminals wearing electronic tags break curfews, the home secretary says.

The probation inspectorate review of tagging in England and Wales comes as a separate critical report from probation officers' union is published.

Napo has detailed 120 examples of mistakes and flaws in the tagging system since the start of 2012.

Theresa May said she was committed to ensuring it was "working properly".

The spotlight on electronic monitoring comes a day after a 15-year-old boy - who was being tagged - was given a life sentence for murdering a student in a row over throwing conkers in north London.

It is an important tool - but of course we want to make sure that it's working properlyTheresa May, Home Secretary

The killer, who cannot be named because of his age, was subject to electronic tag monitoring as part of a rehabilitation order imposed the month before the murder.

Mrs May told BBC News the Ministry of Justice was already looking at the way it tagged people, and that changes would be implemented under new contracts which come into effect in April 2013.

That included looking at the newest technology available, she said.

She added: "It is an important tool - but of course we want to make sure that it's working properly."

The use of electronic tagging has more than doubled in the last six years, with a total of 80,000 people tagged in 2010-11.

The rules that govern breach aren't clear to the people who are subject to the curfew, and it can give them the impression that violations won't be followed upLiz Calderbank, HMIP chief inspector

The inspectorate review of 81 cases found delays were caused by unclear or illegible paperwork and that curfews were often unrelated to the offence committed.

More than a third - 37% - of cases involve serious violations, including damaging equipment or being absent for the entire curfew period.

Liz Calderbank, chief inspector of Her Majesty's Inspectorate of Probation (HMIP), said her main concern was that tagging was "an opportunity missed - it's not being used to challenge offenders' behaviour and help control and change it".

"The rules that govern breach aren't clear to the people who are subject to the curfew and it can give them the impression that violations won't be followed up," she told BBC Radio 4's Today programme.

She said violations were followed up but by the time that happened "a lot of offenders haven't abided by them".

"This all speaks for the need for the curfew to be managed more as a part of a whole programme of working with the offender to change their behaviour," she added.

'Less serious'

In its report, HMIP complained that offenders could break the terms of their curfew for up to two hours before receiving a warning.

There will be the odd time when you are not aware at all that the person's violating the tagProbation officer

And it said being absent for 11 hours and 59 minutes of a 12-hour curfew could trigger only a "a less serious violation".

The Napo report, obtained by Radio 5 live Breakfast, was gathered by probation officers over a period of four months.

It includes a case in which the wrong person was tagged, an example in which a monitoring bracelet was fitted to an artificial leg, and claims that an offender managed to remove his tag and tie it to his dog.

One probation officer from the north of England, who wished to remain anonymous, said with one offender there had been 40 violations before her office was contacted.

"There will be the odd time when you are not aware at all that the person's violating the tag," she said.

"The only time you do become aware is perhaps when they commit an offence in the middle of the night or they're stopped and searched and we get the information from the police."

Failures 'rare'

Harry Fletcher, assistant general secretary of Napo, urged the government to use next year's new contracts as an opportunity to review "the effectiveness and efficiency of tagging to establish its impact, or not, on reoffending before the government embarks on any massive roll-out".

Serco and G4S - the two companies contracted to carry out electronic monitoring in separate parts of the country - defended the current system.

"The vast majority of interactions are completed without incident and equipment failures are extremely rare," said G4S, which is responsible for monitoring about 14,000 people at any one time.

"Surveys undertaken with over 16,000 offenders found that nearly 90% said that being on tag had stopped them offending, and more than 70% had reduced alcohol and drug use as a direct consequence of being on tag."

Serco said it could not comment on individual issues but it insisted it had "a very good record".